If you want to use an existing html file then you could place this in the theme directory and then open the file with a link button making sure the target is your environment url followed by /htmlfile.html
If you are running on localhost:8080 and the file is named testMx.html the link button target should be: http://localhost:8080/testMx.html
You can use the html snippet to load the html file or just copy and paste the html into this widget.
I did try it this way.
I put a 'test.html' under: <myproject folder>/theme
And I put http://localhost:8080/test.html
But I got 404 error shown below. Is there security related to this?